Has salt gotten an unfair shake?

Bret Stetka | NPR

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Sodium is a key element in table salt, and it's also essential for life. It helps regulate our blood volume. It shuttles nutrients into our bodies and brains. It allows our muscles to contract and our nerves to pulse with electricity. Yet for decades, we've been told to avoid it.

Since the 1970s, most major nutrition and health guidelines have cautioned against eating too much sodium, citing associations with high blood pressure that could lead to heart attack and stroke. Recommendations put forth from the Institute of Medicine — now called the National Academy of Medicine — and jointly by the Department of Health and Human Services and the Department of Agriculture in particular have consistently urged us to restrict sodium intake to 2.3 grams per day, equivalent to about 1 teaspoon of salt. Some recommendations even go as low as 1.5 grams for certain people.

High blood pressure does indeed up the risk of cardiovascular disease and mortality in most people. But in many areas of medicine, accepted beliefs often get rewritten as new evidence emerges. Cholesterol is bad for us. No it's not. Saturated fat is surely killing us all. Actually, a little might be okay.

Some health researchers believe it's salt's turn for a reappraisal and point to studies suggesting that more salt doesn't necessarily mean more heart disease.

Participants in a recent study with the highest intake of sodium and potassium actually had significantly lower blood pressure, according to an analysis presented earlier this year at the American Society for Nutrition's Scientific Sessions meeting in Chicago. The group with the lowest blood pressure averaged a daily sodium intake of 3.7 grams a day, far higher than the guidelines suggest.

The findings echo those of a 2016 study published in The Lancet. The largest of its kind, the review looked at sodium intake and blood pressure data in over 130,000 individuals from 49 countries with varying degrees of salt consumption. Low sodium intake was defined as up to 3 grams a day, just shy of the 3.4 grams a day that Americans average. Four and 5 grams a day was considered "moderate" intake, and 7 or more grams a day as "high" consumption.

The authors found that populations with very low sodium intake seemed to have a higher risk of cardiovascular disease and death than those with moderate intake. So did people on high-salt diets, but only those with high blood pressure in the first place. According to the data, moderate to high salt consumption in people with normal blood pressure did not appear to have the dire consequences that might have been presumed.

"In people with normal blood pressure, there doesn't seem to be much of an association between salt intake increases in blood pressure," says lead study author Andrew Mente, a nutritional epidemiologist at McMaster University in Hamilton, Ontario.

"Regardless of your blood pressure — and regardless of whether you have high blood pressure or normal blood pressure — if you bring sodium down to low levels, you get an increased risk in cardiovascular problems," he explains, "Going from high to moderate reduces risk based on our data, but not any further than that."

Mente's analysis was not without its criticisms. Many dietary experts cried foul at potentially inaccurate statistical methods, especially those involving sodium intake estimation.

"Basically they take a single urine sodium measurement each day and then use a formula to estimate sodium intake," explains Dr. Paul K. Whelton, a professor of public health at Tulane University and contributor to many major dietary guidelines. "This 'Kawasaki' formula, as it's called, includes things like age, gender and body weight, all closely related to cardiovascular outcomes and which could be huge confounders of the results," he says. And, he adds, sodium intake estimates based on a single "spot" urine collection are nowhere near as accurate as estimates based on multiple carefully collected 24-hour samples — "which is the gold standard method for estimating sodium intake."

In the 1990s, Whelton led two of the largest investigations to date into the health effects of sodium reduction. Both phases of the Trials of Hypertension Prevention studies reported an association between lower sodium consumption and lower blood pressure.

Whelton also points to a number of studies since then suggesting the same link, including two 2013 meta-analyses published in the British Medical Journal and the well-publicized DASH low-sodium diet research findings, which have in part informed leading society sodium guidelines. The list goes on.

Mente acknowledges that there are plenty of trials reporting a link between high blood pressure and high sodium. Yet he feels such findings are not generalizable to everyone.

"The bottom line is that there is not a shred of evidence whatsoever that low sodium, 2.5 grams per day or lower, is better than average sodium, around 3.5 grams per day, in reducing cardiovascular events or mortality," he says.

Mente says that between his research and that of others, evidence from over 400,000 individuals overwhelmingly indicates that people with moderate sodium consumption have the lowest risk of cardiovascular events and death.

The many guidelines recommending low-sodium diets were based on committee opinions, he points out, and also extrapolate data linking sodium and high blood pressure to heart disease. It's a case of A causes B, and B causes C; but A doesn't necessarily result in C.

In his book The Salt Fix: Why the Experts Got It All Wrong — And How Eating More Might Save Your Life, DiNicolantonio rails against salt orthodoxy, not only arguing against salt restriction but even suggesting we eat more of it.

Citing much of the same research as Mente, he adds that low-sodium diets can actually increase bad cholesterol levels, impair our sugar metabolism and activate our renin-angiotensin system, a network of hormones that regulates blood volume and is a known contributor to hypertension when overactive.

"Many of the authors of these trials and guidelines recommending salt restriction only consider effects on blood pressure, not the other negative effects of a low sodium diet," says DiNicolantonio. "I'm just trying to tell the other side of the story."

DiNicolantonio's patent embrace of salt is, for now, on the fringes of medicine. And as Francesco Cappuccio, a professor of cardiac medicine at the University of Warwick and head of the World Health Organization's Collaborating Centre for Nutrition, explained to NPR, elevated renin-angiotensin activity is the body's normal physiologic response to decreased sodium and not something to worry about.

A number of the experts I spoke with were also quick to point out that for a time, DiNicolantonio was employed as a pharmacist at a Wegmans supermarket and, in 2013, had a paper retracted because of unreliable evidence. He hasn't performed much original research himself — in their eyes not the most glamorous curriculum vitae.

But some of DiNicolantonio's points do make a certain intuitive sense.

"If you look at some of the healthiest diets in the world, the Mediterranean diet, the Japanese diet, these are not low-salt diets," he says. "And many countries with the lowest rates of death due to heart disease eat very salty diets — countries like Japan, France and South Korea."

He feels this is in part because of overall dietary patterns. Sodium delivered through a processed, sugary diet low in nutrients may have much different effects on the body than sodium contained in a healthy diet.

The crux here is other nutrients, potassium in particular. In the 1980s, Whelton was one of the first researchers to report that increasing potassium intake can lower blood pressure. And diets high in potassium-rich foods — sweet potatoes, spinach and an array of beans — are thought to help control blood pressure and mitigate the effects of sodium.

"This may be where we all see eye to eye," says Whelton. "If you give potassium, the body eliminates more sodium and blood pressure comes down. I and many others believe that it may be the sodium-to-potassium ratio that is most important to health."

DiNicolantonio also says that when we talk about salt, we have to specify the type of salt and where it is sourced.

Whereas most table salts contain added sugar and few nutrients, many forms of naturally sourced salt also provide other healthy minerals. These are salts like Celtic sea salt, Himalayan pink salt and DiNicolantonio's favorite, salt sourced from "ancient" oceans, which are dried up prehistoric seas untouched by pollution. With nutrients like potassium and magnesium alongside the usual sodium and chloride, these salts could in theory better help balance sodium intake.

In reality, levels of these other nutrients in natural salts are only a small percentage of recommended daily allowances. But after acquiring a grinder of ancient ocean salt myself, I agree with his other selling point: They simply taste better.

DiNicolantonio feels that seasoning our food with natural salts, or any salt to a degree, could be an enticement to eat a healthier diet. If a dash of sodium means we'll finish that side of potassium-rich sweet potatoes, then our health is better for it.

Mente, who eats around 3.5 grams of sodium a day himself, also believes that the relationship of sodium to potassium and one's general diet is probably key to understanding salt health and that until doctors better understand the link, medical societies should refrain from lowering sodium limits to levels that virtually no populations in the world eat, save a few hunter-gatherer societies.

"The current guidance for low sodium intake is based on an assumed cardiovascular benefit," he argues, and stronger proof should be required before recommending that millions of people reduce their sodium intake to low levels.

"We need to look at other physiological factors. We need to look at the renin-angiotensin system," Mente says. "We need to look at potassium. There's a lot we need to look for."

Bret Stetka is a writer based in New York and an editorial director at Medscape. His work has appeared in Wired and Scientific American and on The Atlantic.com. He graduated from the University of Virginia School of Medicine in 2005. He's also on Twitter: @BretStetka